The present invention generally relates to synchronization techniques for synchronizing the frames of a sending and a receiving multiplexer and for recognizing an out of synch condition. More particularly, the present invention relates to a frame synchronization technique which permits a user to specify the speed at which out of synchronization recognition is to be achieved, including a determination within one frame length, as well as to a technique which permits resynchronization to be achieved expeditiously.
When data from several channels are multiplexed over an aggregate line, it is necessary on the receiving end to divide (i.e. to demultiplex) the data into corresponding channel components so that the data reach proper destinations. The data are governed according to a frame format which permits the demultiplexer to know to which channel a received bit or byte belongs. A common frame format for telecommunications is the DS1 frame format for sending over a T1 aggregate line. In a DS1 frame, a group of one-hundred ninety-three bits of information are sent as a frame at a frame rate of 8 Khz. One of those bits is a DS1 overhead bit which includes DS1 frame synchronization information. The remaining one hundred ninety-two bits are typically divided up into up to twenty-four subaggregates (DS0's). Each subaggregate draws data from the primary frame of a multiplexer which in turn draws data from a secondary frame. The combination of the primary and secondary frames (i.e. the sequence which finally repeats) is known as the composite frame.
Various synchronization techniques ("synchronization technique" including both methods for recognizing loss of synchronization as well as methods for establishing synchronization) for aggregates as well as for composite frames are known to those skilled in the art. A typical synchronization procedure for a composite frame allocates a single "synch bit" in the non-repeating secondary frame. The use of a single bit, however, limits the speed at which the loss of synchronization can be detected regardless of the synchronization scheme, as the synch rate is limited to the rate of the non-repeating frame. As a typical non-repeating frame rate is 25 Hz, and because a minimum of eight incorrect bits are usually required to establish an out of frame condition, where a single secondary frame synch bit is used, a minimum of 320 milliseconds would be required to recognize an out of synch condition. Of course, such a relatively large time of misdirecting data is undesirable. Moreover, in reestablishing synchronization, it is often necessary for several frames of information to be utilized before the synch bits can be identified.
Other synchronization schemes permit loss of synchronization to be detected more quickly. Such schemes allocate additional bandwidth to synchronization. In these schemes, a plurality of adjacent synch bits are formed into a word (sometimes designated as a "preamble") which typically permits an out of synch determination to be made within the time frame of two composite frames. Similarly, the reestablishing of synchronization is relatively quickly accomplished.
While synchronization of the aggregate (DS1 frame) itself is not within the scope of the instant invention, it is important to note that when the aggregate line loses synchronization, the synchronization of all of the sub-aggregates is lost. Thus, upon regaining aggregate synchronization, it is necessary to quickly synchronize all of the subaggregate frames, including both the primary and secondary frames.